* Pros and cons of salt-and-pepper cabling
The various Wi-Fi vendors have different ways of handling redundancy in the event that an access point (AP) or WLAN controller should fail. One variable to consider when designing a controller-based network for high availability are the applications you intend to run.
For example, it’s common practice in some controller-based architectures to wire APs that are physically near to one other to different controllers at the back end in an alternating fashion – often called a “salt and pepper” layout. This makes basic sense: If APs in a given cell are all connected to one common controller and that controller fails, then that whole cell coverage area is down while the controller fails over to a backup.
By contrast, a salt-and-pepper layout would allow a neighboring AP to keep running and perhaps increase its power output momentarily using dynamic transmission power control, or DTPC, to “fill in” temporarily for an AP connected to the downed controller.
On the other hand, the failover process can take as long as nine seconds, depending on the controller-based system. As a result, some voice implementations don’t work well with the salt-and-pepper layout. Vocera, for example, advises avoiding this type of layout with its low-power voice-over-WLAN wearable badges.
All this being said, some controller-based vendors such as Trapeze use a virtual clustering technology whereby multiple controllers function as “one big controller” at the back end. As such, any controller can take over for any other within about 30 milliseconds, according to the vendor.
Trapeze and many others also have local bridging capabilities for voice, meaning that voice sessions, once authenticated, don’t need to hit the controller again. This works but might require you to push at least the voice VLAN configuration out to all your APs, which is a more complex undertaking.
Dual-homing APs to two back-end controllers, which authenticate each client, is another option that should keep sessions running. However, it will require the use (and cost) of additional switch ports in the wiring closet.
These are just a few technical and cost issues to consider when planning a high-availability environment in a controller-based Wi-Fi network. If you are planning to run special voice or location applications, be sure to check with the maker of the client application or device for the specs and configurations they recommend in the various vendor environments.
